Field of the Invention
This invention relates generally to friction stir operations such as friction stir welding (FSW), friction stir processing (FSP), friction stir mixing (FSM), friction surfacing, friction hydro pillar processing, friction stir forming, friction extrusion, and friction stir spot welding (FSSW) (and hereinafter referred to collectively as “friction stir operations”). More specifically, the present invention relates to the use of control algorithms that utilize control of effort (e.g. torque) in order to improve control of the friction stir operations.
Description of Related Art
Friction stir welding is a technology that has been developed for welding metals and metal alloys. The FSW process often involves engaging the material of two adjoining workpieces on either side of a joint by a rotating stir pin. Force is exerted to urge the pin and the workpieces together and frictional heating caused by the interaction between the pin, shoulder and the workpieces results in plasticization of the material on either side of the joint. The pin and shoulder combination or “FSW tip” is traversed along the joint, plasticizing material as it advances, and the plasticized material left in the wake of the advancing FSW tip cools to form a weld. The FSW tip can also be a tool without a pin but only a shoulder for processing of another material through FSP.
FIG. 1 is a perspective view of a tool being used for friction stir welding that is characterized by a generally cylindrical tool 10 having a shank 8, a shoulder 12 and a pin 14 extending outward from the shoulder. The pin 14 is rotated against a workpiece 16 until sufficient heat is generated, at which point the pin of the tool is plunged into the plasticized workpiece material. Typically, the pin 14 is plunged into the workpiece 16 until reaching the shoulder 12 which prevents further penetration into the workpiece. The workpiece 16 is often two sheets or plates of material that are butted together at a joint line 18. In this example, the pin 14 is plunged into the workpiece 16 at the joint line 18.
FIG. 2 is a cross-sectional view of a typical tool 10, but should not be considered as limiting. A collar 22 is shown gripping both the shank 8 and the FSW tip 24, wherein the FSW tip is comprised of the shoulder 12 and the pin 14. As the tool 10 is rotated, torque (i.e. effort) is transmitted from the rotating shank 8 to the collar 22 and then to the FSW tip 24.
Referring to FIG. 1, the frictional heat caused by rotational motion of the pin 14 against the workpiece material 16 causes the workpiece material to soften without reaching a melting point. The tool 10 is moved transversely along the joint line 18, thereby creating a weld as the plasticized material flows around the pin 14 from a leading edge to a trailing edge. The result is a solid phase bond 20 at the joint line 18 that may be generally indistinguishable from the workpiece material 16 itself, in contrast to welds using other conventional technologies. It is also possible that the solid phase bond 20 is superior to the original workpiece material 16 because of the mixing that occurs. Furthermore, if the workpiece material is comprised of different materials, the resulting mixed material may also be superior to either of the two original workpiece materials.
It is observed that when the shoulder 12 contacts the surface of the workpieces, its rotation creates additional frictional heat that plasticizes a larger cylindrical column of material around the inserted pin 14. The shoulder 12 provides a forging force that contains the upward metal flow caused by the rotating tool pin 14.
During friction stir welding, the area to be welded and the tool 10 are moved relative to each other such that the tool traverses a desired length of the weld joint. The rotating friction stir welding tool 10 provides a continual hot working action, plasticizing metal within a narrow zone as it moves transversely along the workpiece materials 16, while transporting metal from the leading edge of the pin 14 to its trailing edge. As a weld zone cools, there is typically no solidification as no liquid is created as the tool 10 passes. It is often the case, but not always, that the resulting weld is a defect-free, recrystallized, fine grain microstructure formed in the area of the weld.
Travel speeds of friction stir tools change depending upon the specific type of friction stir operation being performed, the application and the material being processed. Some examples of travel speeds are over 1 m/min with rotation rates of 200 to 3000 rpm. These rates are only examples and should not be considered to be limiting the operation of the present invention. Temperatures reached are usually close to, but below, solidus temperatures. Friction stir welding parameters are a function of a material's thermal properties, high temperature flow stress and penetration depth.
Friction stir welding has several advantages over fusion welding because 1) there is no filler metal, 2) the process can be fully automated requiring a relatively low operator skill level, 3) the energy input is efficient as all heating occurs at the tool/workpiece interface, 4) minimum post-weld inspection is required due to the solid state nature and extreme repeatability of FSW, 5) FSW is tolerant to interface gaps and as such little pre-weld preparation is required, 6) there is typically no weld spatter to remove, 7) the post-weld surface finish can be exceptionally smooth with very little to no flash, 8) there is often no porosity and oxygen contamination, 9) there is little or no distortion of surrounding material, 10) no operator protection is required as there are no harmful emissions, and 11) weld properties are often improved. Throughout this document, friction stir operations will be considered to include all processes that can be performed using a friction stir tool, including but not limited to friction stir welding, friction stir processing, friction stir spot welding and friction stir mixing.
In friction stir welding, the temperature of the process zone or friction stir zone affects the properties of the resulting weld and has a dramatic effect on tool life such as in PCBN (polycrystalline cubic boron nitride) tools. This tool example should not be considered to be limiting. An active control system that changes process parameters to control weld temperature is desirable.
Controlling the weld temperature throughout the length of the weld is an important undertaking because weld properties, such as fracture toughness and corrosion resistance, vary with weld temperature. If specified properties are desired throughout the weld, the weld temperature must be adjustable and in control throughout the length of the weld.
The short tool life of PCBN tools limits the application of friction stir processing (FSP) in steels and other high softening temperature (or high melting temperature) materials. Controlling tool temperature should increase the tool life of PCBN tools because some temperature problems can cause damage. For example, if the temperature is too low, the tool is overstressed by forces that increase as tool temperature decreases. In contrast, if the tool temperature is too high, PCBN tools fail quickly because of chemical wear. High temperature can also cause creep in the locking collar allowing the PCBN insert to rotate. Point stresses will then likely exist at the corners of the insert on cooling that may lead to cracking and failure.
The first efforts to control weld temperature used passive control techniques. Researchers considered that the weld temperature was proportional to weld pitch (spindle speed)/(travel speed) or various “pseudo heat indexes” which are functions of spindle speed and travel speed. Passive control techniques assume that the process has reached a self-limiting equilibrium condition.
Passive control techniques are not adequate for temperature control because equilibrium conditions may not exist along the length of a weld. Causes of temperature changes along the length of the weld include: inadequate cooling of the tool or backing plate; changes in the thermal boundary condition; and insufficient time to reach equilibrium. Passive control techniques are not versatile because they do not adjust for process disturbances.
FIG. 3 is a block diagram of an active control system for a friction stir welding machine. A friction stir welding machine includes a friction stir welding tool that is coupled to a spindle, which in turn is coupled to a spindle motor. The friction stir welding machine also includes a surface for clamping or supporting workpieces to be friction stir welded. A friction stir welding machine can be controlled by the active control system to thereby perform friction stir welding.
FIG. 4 shows that the prior art also teaches a two-stage control model that contains an inner loop that controls the spindle speed to keep power constant and an outer loop for setting the desired power based on weld temperature. FIG. 4 is a block diagram showing a close-up of the inner loop of FIG. 3.
FIG. 3 shows a temperature control algorithm where T is temperature, w is spindle speed, M is torque and P is power.
FIG. 5 is provided as a block diagram of a close-up view of a stir zone from FIGS. 3 and 4. A plant is the combination of the spindle motor and the stir zone. For the outer loop the reference is desired temperature, the controlled variable is temperature and the manipulated variable is power. For the inner loop the reference is commanded power and the controlled variable is power.
The prior art shown in FIG. 3 teaches that the inner loop adjusts spindle speed to maintain constant power. It is critical to the present invention to understand this aspect of the prior art. The purpose of the inner loop is to maintain a desired power provided to the stir zone.
The relationship between power and spindle speed is given by:P=ωM  Equation (1)where P is power, ω is spindle speed in radians/s and M is torque. Power control by adjusting spindle speed uses torque feedback and Equation (1) to solve for the spindle speed required to produce the desired power. A block diagram for power control by adjusting spindle speed is shown in FIG. 4 where M(filtered) is the filtered value of the reported torque. This control scheme assumes that torque is constant during each PLC time step. Slew limits define the maximum acceleration of the spindle. Slew limits are used to prevent the system from going unstable due to noise in torque feedback. If slew limits are set too high, the system will amplify noise in the torque feedback and become unstable.
Results for power control by adjusting spindle speed with a slew rate of 0.83 RPM/s are shown in FIG. 6C. When power control is enabled, the torque is high and the RPM is low. As the weld progresses the plate heats and softens causing a decrease in torque. As torque decreases, the RPM increases, thereby maintaining constant power. Large spikes in power persist throughout the weld as shown. Power spikes occur because the spindle motor attempts to accelerate the spindle instantaneously to achieve the commanded RPM. Adjusting spindle speed to control power results in large power spikes throughout the weld. The average of these power spikes is the desired power value.
It would be an advantage over the prior art to provide an improved active control system that does not use spindle speed to control power.